Method and apparatus for a portable communication device to identify its own location

ABSTRACT

A method and apparatus for a portable communication device, such as a mobile radio terminal in a CDMA network, to identify its geographic location. A communication network comprises multiple base stations which transmit pilot signals, where the pilot signals of one base station are offset from the pilot signals of other base stations by integral multiples of a substantially fixed duration. Associated sync channel messages are also transmitted from which the geographic location of the base stations may be extracted. A portable communication device identifies its location by calculating the delay between the arrival time of a pilot signal from a primary base station and the arrival times of pilot signals from at least two other base stations. For each pair of base stations corresponding to one of the calculated delays, the portable communication device initially specifies its location as being on a selected one of three curves. The current location of the portable communication device is identified based upon points of intersection of the selected curves. The identified location may be stored in, transmitted from, or displayed at the portable communication device.

This is a continuation of application Ser. No. 08/338,911 filed on Nov.14, 1994, now U.S. Pat. No. 5,646,632.

FIELD OF THE INVENTION

The present invention relates generally to radio communication systems,such as CDMA cellular telephone networks, and, in particular, to amethod and apparatus for a portable communication device to identify itsown location within such a system.

BACKGROUND OF THE INVENTION

Mobile or portable radio terminals in cellular networks permit users ofsuch radio terminals to move from place to place within a geographicarea and maintain or establish a communication channel with basestations in the network. The base stations are typically connected to anexisting telephone network, thereby providing access to a largertelecommunication system.

Situations often arise where it is desirable to determine the locationof a mobile or portable radio terminal. Such situations may arise, forexample, in the event of a medical or safety emergency so that help maybe dispatched. Similarly, it may be desirable to track a radio terminalas it moves within a particular geographic area for the purpose oftracking assets or people that are travelling with the radio terminal.

Several techniques are known for locating a mobile or portable radioterminal. Loran-C receivers, for example, use a radio pulsed navigationsystem. Another proposed technique would use the Global PositioningSystem (GPS) in conjunction with a cellular telephone network. The GPS,however, is a satellite based navigation and positioning system and doesnot perform well inside buildings. Furthermore, the system would requirethat a user have both a GPS receiver as well as cellular telephoneservice.

Another technique for locating a mobile or portable radio terminal isdisclosed in U.S. Pat. No. 5,293,645. The disclosed method involvescompiling relative propagation delays for synchronized timing signalstransmitted from a plurality of base stations in a cellular network andreceived by the radio terminal to be located. The radio terminal thentransmits the compiled information to a base station or other unit inthe network where the location of the radio terminal is identified. Onefeature of the disclosure of the aforementioned patent is that in orderto identify the location of the radio terminal, the radio terminal musttransmit information to a unit in the network that is external to theradio terminal. Furthermore, according to the disclosure of theaforementioned patent, the user of the radio terminal does not haveimmediate access to its location.

SUMMARY OF THE INVENTION

The present invention discloses a method and apparatus for identifyingthe location of a mobile or portable radio terminal in a cellulartelephone network that does not require that the radio terminal transmitinformation to other units in the network. According to the principlesof the present invention, a radio terminal in the network can determineits own location in a manner that uses the existing framework of codedivision multiple access (CDMA) technology and thus requires nomodification of current CDMA protocols.

The portable communication device of the present invention may suitablycomprise a receiver and a transmitter. It may also comprise a relativedelay calculation unit, coupled to the receiver, for computing delaysbetween the arrival time of a pilot signal transmitted from a primarybase station and the arrival times of pilot signals transmitted from atleast two other base stations. The device may also have a memory unit,coupled to the receiver, for storing information indicative of thelocation of the primary base station and the other base stations whosepilot signals were received. In addition, the device may suitablyinclude a processor, coupled to the relative delay calculation unit andthe memory unit, where the processor is programmed to identify thelocation of the portable communication device.

Other features and advantages of the present invention will be readilyapparent by reference to the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cellular radio network in which thepresent invention is particularly advantageous.

FIG. 2 is a block diagram of a mobile according to the principles of thepresent invention.

FIG. 3 illustrates a technique for identifying the location of a mobileaccording to the principles of the present invention.

FIG. 4 is a flow chart illustrating the steps of identifying thelocation of a portable communication device according to the principlesof the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a cellular radio network 100 in which thepresent invention is particularly advantageous. The network 100preferably is a code division multiple access (CDMA) network, whichincludes a plurality of base stations 110, 111 and 112. Although onlythree base stations are shown in the network 100 of FIG. 1, a typicalCDMA network may have many more base stations. The base stations 110-112are connected via a communication link, as is well known in the art.Also, as is well known in the art, each base station in the CDMA network100 generates and transmits pilot signals or reference timing signals ina synchronous manner within the tolerances of the synchronizationtechnique. The pilot signals or reference timing signals are knownperiodic sequences of bits, which may be modulated, for example,according to the CDMA standard. The pilot signals transmitted by onebase station, such as the base station 110, are offset in time byintegral multiples of a fixed duration with respect to the pilot signalstransmitted by other base stations in the network 100. The fixedduration may be referred to as a staggered pilot signal transmissiontime and may be represented by the symbol Δ. The staggered pilot signaltransmission time is substantially constant within the tolerances of thesystem. Thus, the start of a sequence of bits representing a pilotsignal transmitted by one base station is offset from the start ofsequences of bits representing pilot signals transmitted by other basestations by an integral multiple of the staggered pilot signaltransmission time. The time offset allows the pilot signals of one basestation to be distinguished from those of other base stations.

The base stations 110-112 provide a network by which a mobile orportable radio terminal 130 within the network 100 can link up with atelecommunication network. In the present specification, a portablecommunication device, such as the mobile or portable radio terminal 130,will be generically referred to as a mobile. Pilot signals transmittedby the base stations 110-112 and received at the mobile 130 arerepresented in FIG. 1 by the lines 120-122, respectively. It should alsobe noted that the staggered pilot signal transmission time willtypically be several orders of magnitude greater than the propagationtime of the pilot signals from the base stations 110-112 to the mobile130. The staggered pilot signal transmission time Δ may be storedpermanently in a memory in the device 130.

FIG. 2 is a block diagram of the mobile 130 according to the principlesof the present invention. The mobile 130 preferably comprises a keypad201, a handset or other device which allows a user to instruct themobile 200 to determine or identify its location. Alternatively, themobile 130 may be designed to respond to an instruction originating fromother sources in the network. In yet a further embodiment, the mobile130 may be programmed to identify automatically its location on aperiodic basis.

The keypad 201 is coupled to a receiver 210 which is tuned to detectpilot signals from the base stations. Upon receiving the instruction toidentify the location of the mobile 130, the mobile 130 searches forpilot signals. In a preferred embodiment, the mobile 130 locks onto thestrongest pilot signal. The base station whose pilot signal is strongestthen serves as a primary base station, although other base stations,such as the base station whose pilot signal is received first, may alsoserve as the primary base station. The mobile 130 continues to searchfor pilot signals from other base stations as well.

The mobile 130 further comprises a relative delay calculation unit 215which is coupled to the receiver 210. The relative delay unit 215includes a timer or clock 214 which records the time at which each pilotsignal is received. Once pilot signals from at least three base stationshave been received, the relative delay calculation unit 215 computes thedelay between the arrival time of the pilot signal transmitted from theprimary base station and the arrival times of the pilot signalstransmitted from the other base stations. Alternatively, the timer 214may begin running when the pilot signal from the primary base station isreceived. As subsequent pilot signals from other base stations arereceived, the relative delay calculation unit records the time that haspassed since the pilot signal from the primary base station wasreceived. The relative delay calculation unit 215 thus determines thedelay between the arrival time of the pilot signal transmitted from theprimary base station and the arrival time of the pilot signalstransmitted from at least two other base stations.

The mobile also extracts information contained in a sync channel messageassociated with each base station whose pilot signal is received.Specifically, the mobile 130 would extract information indicating thegeographical location or coordinates of each base station whose pilotsignal is received. The geographical coordinates of the base stationsmay be stored, at least temporarily, in a base station location memoryunit 216 for further use as explained below.

A mobile location unit 220 is coupled to the relative delay calculationunit 215 and the base station memory 216 so as to receive signalsindicating the calculated delays between arrival times of the pilotsignals and the coordinates of the base stations. The mobile locationunit 220 and the relative delay calculation unit 215 may be implementedby an appropriately programmed microprocessor. For each pair of basestations corresponding to one of the computed delays, the mobilelocation unit 220 specifies the location of the mobile 130 as beinglocated on a selected one of three curves, as illustrated in FIG. 3.

FIG. 3 illustrates a technique for identifying the location of a mobileaccording to the principles of the present invention. FIG. 3 shows themobile 130 which has received pilot signals from at least three basestations labelled A, B and C. For the purpose of illustration, it willbe assumed that the base station A is the primary base station.Furthermore, it will be assumed that the mobile 130 has calculated adelay T_(AB) between the arrival times of the pilot signals transmittedfrom the base stations A and B respectively. It will also be assumedthat the mobile 130 has calculated a delay T_(AC) between the arrivaltimes of the pilot signals transmitted from the base stations A and Crespectively.

With respect to the pair of base stations A and B corresponding to thedelay T_(AB), the first curve is defined by a straight line zz' whichbisects and runs perpendicular to a line drawn from the base station Ato the base station B. The second and third curves, vv' and ww',respectively, are defined as the points (x, y) that satisfy thefollowing equation: ##EQU1## where k and l are the horizontal andvertical coordinates, respectively, of the midpoint between the linethat intersects the base stations A and B. Also in the above equation,the term `a` equals the product of the delay T_(AB) and the radiopropagation velocity, divided by a factor of two. In the above equation,the term `b² ` equals the product of a² and (e² -1), where the term `e`equals the distance between the point (k, l) and the base station A,divided by `a`.

The mobile location unit 220 would then specify the mobile 130 as beinglocated on a selected one of the three curves zz', vv' or ww', basedupon the relative values of the delay T_(AB) and the actual durationΔ_(AB) between the transmission times of the pilot signal from the basestation A and the pilot signal from base station B. Although the mobile130 does not know the actual duration Δ_(AB) between the transmissiontimes, it can estimate that value by determining which integral multipleof the staggered pilot signal transmission time Δ is closest to thevalue T_(AB). Thus, if T_(AB) is equal to Δ_(AB), then the mobilelocation unit 220 identifies the mobile 130 as being located on thecurve zz'. If T_(AB) is greater than Δ_(AB), then the mobile locationunit 220 identifies the mobile 130 as being located on the hyperbolavv'. Finally, if T_(AB) is less than Δ_(AB), the mobile location unit220 identifies the mobile 130 as being located on the curve ww'.

In a preferred embodiment, assuming that the pilot signals of thevarious base stations in the network are transmitted at the same power,the mobile location unit 220 would specify the mobile 130 as beinglocated on one of the three curves based upon the relative strength orpower of the received pilot signals. Specifically, the mobile locationunit 220 would identify the mobile 130 as being located on the hyperbolawhich is closer to the base station whose received pilot signal wasstronger. If the strength of the received pilot signals were the same,within the tolerances of the system, then the mobile location unit 220would identify the mobile 130 as being located on the line zz'. For thepurpose of illustration, it will be assumed that the mobile locationunit 220 specifies the mobile 130 as being located on the selected curvevv'.

The mobile location unit 220 would then specify, in a similar manner,the mobile 130 as being on a selected one of three similar curves, usingthe delay T_(AC) between arrival times of the pilot signals transmittedfrom the base stations A and C, and using the coordinates of the basestations A and C.

With respect to the pair of base stations A and C corresponding to thedelay T_(AC), the first curve is defined by a straight line rr' whichbisects and runs perpendicular to a line drawn from the base station Ato the base station C. The second and third curves, ss' and tt',respectively, are defined as the points (x, y) that satisfy thefollowing equation: ##EQU2## where m and n are the horizontal andvertical coordinates, respectively, of the midpoint between the linethat intersects the base stations A and C. Also in the above equation,the term `f` equals the product of the delay T_(AC) and the radiopropagation velocity, divided by a factor of two. In the above equation,the term `g² ` equals the product of f² and (h² -1), where the term `h`equals the distance between the point (m, n) and the base station A,divided by `f`.

As before, the mobile location unit 220 would then specify the mobile130 as being located on a selected one of the three curves rr', ss' ortt' based upon the relative values of the delay T_(AC) and the actualduration Δ_(AC) between the transmission times of the pilot signal fromthe base station A and the pilot signal from base station C. The mobile130 can estimate the value of Δ_(AC) by determining which integralmultiple of the staggered pilot signal transmission time Δ is closest tothe value T_(AC). Alternatively, in the preferred embodiment, the mobilelocation unit 220 would specify the mobile 130 as being located on oneof the three curves rr', ss' or tt', based upon the relative strength orpower of the received pilot signals. Again, for the purpose ofillustration, it will be assumed that the mobile location unit 220specifies the mobile as being located on the selected curve ss'.

Finally, the point of intersection between the two selected curvesrepresents the identified current location of the mobile 130. Thus, inthe example illustrated in FIG. 3, the mobile location unit 220 wouldidentify the geographic location of the mobile 130 as being at the pointof intersection between the selected curves vv' and ss'.

FIG. 4 is a flow chart conveniently illustrating the steps ofidentifying the location of a portable communication device, such as themobile 130, according to the principles of the present invention. Asshown in step 400, a plurality of at least three base stations, such asthe base stations A, B and C in FIG. 3, transmit pilot signals in themanner described above. Next, as indicated by step 410, the portablecommunication device receives pilot signals from at least three of thebase stations. Referring again to FIG. 3, the mobile 130 would thusreceive pilot signals from the base stations A, B and C. The portablecommunication device then computes the delays between the arrival timeof a pilot signal transmitted from one of the base stations and thearrival times of pilot signals transmitted from at least two other basestations, as shown in step 420. With reference to the exampleillustrated in FIG. 3 and discussed above, the mobile 130 would computethe delays T_(AB) and T_(AC). As illustrated by step 430, the portablecommunication device also receives information indicative of thelocation of the base stations whose pilot signals were received. Againreferring to FIG. 3, the mobile 130 would receive information indicativeof the location of the base stations A, B and C. As explained above,this information may be obtained from sync channel messages associatedwith each pilot signal. Next, as indicated by step 440, for each pair ofbase stations corresponding to one of the computed delays, thegeographic location of the portable communication device is specified asbeing on a selected one of three curves, thereby resulting in aplurality of selected curves. Referring again to the example illustratedin FIG. 3, the mobile 130 specifies its location as being on theselected curve vv' from among the three curves zz', ww' and vv' withrespect to the pair of base stations A and B. Similarly, the mobile 130specifies its location as being on the selected curve ss' from among thethree curves rr', ss' and tt' with respect to the pair of base stationsA and C. The geographic location of the portable communication device isidentified based upon a point or points of intersection of the selectedcurves, as shown in step 450. Thus, in the example illustrated in FIG.3, the mobile 130 would identify its geographic location as being at thepoint of intersection of the curves vv' and ss'. Finally, as shown instep 460, the location of the portable communication device may bestored in memory in the portable communication device, displayed on adisplay screen associated with the portable communication device, ortransmitted to a remote location.

In particular, once the mobile location unit 220 has identified thelocation of the mobile 130, the identified location may be stored in amobile location memory 240 for subsequent retrieval. In anotherembodiment, a transmitter 211, coupled to the mobile location unit 220,would automatically transmit the identified location to a remotelocation, such as a central tracking office, or to a location specifiedby the user according to information entered via the keypad 201. In yetanother embodiment, the identified location would be displayed on anelectronic or other display screen 230 coupled to the mobile locationunit 220. The location would be displayed, for example, either as textor in the form of an electronic map. This embodiment would allow thelocation of the mobile to be made known to the user of the mobile.Applications of the present invention include, for example, navigation,the tracking of assets or people, and fleet management.

It should be noted that, according to the principles of the presentinvention, the absolute or actual delay between the transmission timeand the arrival time of a pilot signal sent by any particular basestation need not be determined or calculated. Rather, only the relativedelays between arrival times are used. It should also be noted that themobile 130 may need to receive several sequences of pilot signals inorder to properly establish the sequence in which the base stations aretransmitting their respective pilot signals. Receiving several sequencesof pilot signals also allows the mobile 130 to use average delaysbetween times of arrival. Thus, in a CDMA system, in which the pilotrepetition rate is typically 37.5 Hertz, reception of pilot signals overa period of approximately eleven seconds appears adequate to obtainreliable results. Also, the pilot signals of more than three basestations may be used to improve the identification of the mobile'slocation. In such a situation, the selected curves for each computeddelay and corresponding pair of base stations may intersect at more thanone point due to the tolerances of the system. The mobile then may belocated within the vicinity of the intersecting points by using aweighted function to estimate the precise location of the mobile.

Although the present invention has been described with reference tospecific embodiments, it will be appreciated that other arrangementswithin the spirit and scope of the present invention will be readilyapparent to persons of ordinary skill in the art. For example, it willbe understood that different sets of curves may be used to specify andidentify the location of the portable communication device. The presentinvention is, therefore, limited only by the appended claims.

We claim:
 1. Apparatus for a portable communication device to locallyidentify its own location within a cellular telephone networkcomprising:a receiver for receiving a first pilot signal transmittedfrom a first base station and at least second and third pilot signalstransmitted from at least second and third base stations; a relativedelay calculation unit coupled to the receiver for computing delaysbetween an arrival time of the first pilot signal and arrival times ofthe second and third pilot signals; and a processor coupled to therelative delay unit, said processor programmed to determine the locationof the portable communication device.
 2. The apparatus of claim 1further comprising a memory coupled to the receiver for storinginformation indicative of the location of the first, second and thirdbase stations.
 3. The apparatus of claim 1 wherein the network is a CDMAnetwork and the pilot signals further comprise known periodic sequencesof bits modulated according to the CDMA standard with said pilot signalsoffset by integral multiples of a predetermined duration with respect toone another.
 4. The apparatus of claim 3 wherein said offsets are storedin a memory.
 5. The apparatus of claim 1 wherein the relative delaycalculation unit further comprises a clock circuit which records thetime at which each pilot signal is received.
 6. The apparatus of claim 1further comprising a keypad operatively connected to the processor toallow the processor to be instructed to determine the location of theportable communication device.
 7. The apparatus of claim 1 wherein theprocessor is operatively programmed to respond to a remotely transmittedinstruction received from a remote source in the network to determinethe location of the portable communication device.
 8. The apparatus ofclaim 7 further comprising a transmitter operative to transmit thelocation of the portable communication device to said remote source inresponse to the remotely transmitted instruction.
 9. The apparatus ofclaim 1 wherein said processor is programmed to automatically identifyits location on a periodic basis.
 10. The apparatus of claim 1 whereinthe processor stores the location of the portable communication devicein a memory for subsequent retrieval.
 11. The apparatus of claim 1further comprising a display to display the location of the portablecommunication device.
 12. The apparatus of claim 1 wherein said receiverfurther operates to receive a sync channel message includinggeographical location information for each base station for which acorresponding pilot signal is received.
 13. The apparatus of claim 1further comprising circuitry for determining the relative strengths ofthe received pilot signals and the location of the portablecommunication device is determined based upon said relative strengths.14. The apparatus of claim 1 wherein average delays between the arrivaltime of the first pilot signal and the arrival times of the second andthird pilot signals are computed over a predetermined period of time.15. A method for a portable communication device to locally identify itsown location within a cellular telephone network comprising the stepsof:receiving pilot signals and locking onto a first pilot signaltransmitted from a first base station; recording the time at which thefirst pilot signal is received; locking onto second and third pilotsignals transmitted from a first base station; recording arrival timesfor the second and third pilot signals; computing a first delay betweenthe arrival of the first and second signals and a second delay betweenthe arrival of the first and third signals; and determining the locationof the portable communication device based upon the first and seconddelays.
 16. The method of claim 15 further comprising the step ofextracting information indicative of the geographical location of basestations transmitting received pilot signals from a sync channel messageassociated with each pilot signal.
 17. The method of claim 15 furthercomprising the step of determining the relative strength of the first,second, and third pilot signals.
 18. The method of claim 17 furthercomprising the step of transmitting the location of the portablecommunication device to a remote location.
 19. The method of claim 15further comprising the steps of:recomputing the first delay and thesecond at a later time; and computing an average first delay and anaverage second delay.